The top Republican on the House Foreign Affairs Committee said Tuesday despite the Obama administration’s willingness to “move closer and closer to Iranian positions,” ongoing talks over the Islamic republic’s disputed nuclear program have hit a dead end.

“Negotiations, now into their second extension, appear to be stalemated,” said Rep. Ed Royce, California Republican, who hammered the White House for failing to “to explain a single change as to how it will negotiate differently within Iran over the coming months.”

His comments marked the latest in a mounting fight between Congress and the administration over how to proceed with the talks, which have carried on in secret between negotiators from Iran, the U.S. and five other world powers over the past year.

A core question underpinning the talks at the moment centers on whether Tehran is allowing U.N. weapons inspectors sufficient access to Iranian nuclear sites to confirm that the nation is not pursuing a nuclear weapon. The U.N.’s nuclear watchdog group — the International Atomic Energy Agency (IAEA) — has given mixed signals on the question recently.

A monthly Iran report that the IAEA issued last week maintained that Tehran is honoring its commitment not to expand atomic activities that could be used to make weapons while it negotiates with the U.S. and other world powers.

Days after the report became public, however, IAEA Director General Yukiya Amano cast doubt over Iran’s activities by suggesting Tehran may still be secretly diverting some material from civilian nuclear activities to a weapons program.

IAEA inspectors have been “able to verify the non-diversion of nuclear material declared to us by Iran,” Mr. Amano told an audience at the University of Indonesia on Jan. 23. “But we are not in a position to provide credible assurance about the absence of undeclared nuclear material and activities in Iran.”

As a result, the IAEA director said, inspectors “are not in a position” to “conclude that all nuclear material in Iran is in peaceful activities.”

Both Mr. Royce and Rep. Eliot Engel, New York Democrat and the committee’s ranking member, voiced support for legislation to increase economic sanctions on Iran ahead of an extended June deadline in the nuclear talks.

President Obama has threatened to veto any new sanctions legislation, saying it could scuttle the talks and heighten the risk of a military showdown with Iran. Lawmakers argue the administration has been too lenient over the past year by easing current sanctions in exchange for Iran’s cooperation in the talks.

Western powers originally leveled the sanctions after years of suspicion that Iran was clandestinely pursuing nuclear weapons in violation of U.N. regulations. Tehran has long denied this, claiming its nuclear program is for purely peaceful purposes.

Mr. Royce asserted Tuesday that Iranian leaders are guilty of “duplicity and militancy” and that Tehran has been “advancing its nuclear program” all while putting “on a good face in a European negotiating room” with the U.S. and other world powers.

“A final agreement would free Iran of sanctions — which has driven it to the negotiating table — while allowing it to maintain a mutually defined [nuclear] enrichment program, to be treated like any other non-threshold nuclear state,” Mr. Royce said in prepared remarks. “That best case would leave Iran as a threshold nuclear state.”

Robert J. Einhorn, a senior fellow at the Brookings Institution in Washington, defended the Obama administration’s handling of the nuclear talks during Tuesday’s hearing, asserting that stiff economic sanctions currently remain in place against Tehran.

The argument that an initial agreement reached in the nuclear talks last year has “led to an erosion of sanctions and reduced economic pressure against Iran is sharply contradicted by the facts,” said Mr. Einhorn, who once served as assistant secretary of state for nonproliferation under President Bill Clinton.

“Although companies around the world have held discussions with Iran in the hope of entering or re-entering the Iranian market, they are taking a very cautious approach, waiting for a nuclear deal to be concluded and sanctions to be removed before taking the risk of signing new contracts,” he said in prepared remarks.

He added that the easing of sanctions under the ongoing talks has meant that Iran has received $700 million per month of its own oil revenues that had been frozen in overseas restricted accounts.

“These repatriated funds are a small fraction of the losses Iran continues to suffer from the sanctions,” Mr. Einhorn said. “The Treasury Department estimates that in 2014, oil sanctions alone deprived Iran of about $40 billion in oil revenues.”